Implements API validator and uses it in user endpoint

internal/model/user.go

@@ -1,7 +1,7 @@
 package model
 
 type User struct {
-	ID        string `json: id`
-	FirstName string `json: "firstName"`
-	Email     string `json: "email"`
+	ID        string `json:"id" validate:"required"`
+	FirstName string `json:"firstName" validate:"required"`
+	Email     string `json:"email" validate:"required,email"`
 }